Potential of Ferric and Polyaluminium Coagulants for Nanofiltration Pretreatment

Abstract

The conventional chemical treatment of soft, cold and humic surface water was studied on a pilot scale, and fouling indexes were used to evaluate the suitability of the pretreated water for nanofiltration. The recommended values for fouling indexes were easily achieved. The best modified fouling index results were about 4 s/12 with two ferric salts and about 5 s/12 with polyaluminium chloride. Optimal flocculation pH and coagulant dosages for the modified fouling index were also suitable for minimizing organic matter and turbidity at the same time. In addition to removing organic carbon, nanofiltration can be applied to decrease ionic substances such as coagulant residues. Chemical treatment reduces organic matter of higher molar mass efficiently, but organic residue together with residual iron or aluminium may induce fouling. In nanofiltration pilot-scale studies at a waterworks using polyaluminium chloride, residual aluminium was observed to increase the fouling of membranes. The original water flux through NF membranes was, however, recovered by chemical cleaning, indicating the fouling was reversible.